As part of the effort to establish an archeointensity variation curve for Mesoamerica, 13 archeologically wellidentified pottery samples belonging to the Ocozocoautla site (Chiapas) were studied. Analyzed samples consist of 'ofrenda type' pottery fragments found in several caves. Three archeological intervals are involved: 450-100 B.C., 200-550 A.D. and 550-900 A.D. The Thellier method in its modified form was applied to small fragments previously embedded in salt pellets. Raw intensity values were further corrected for cooling rate effects. The common time-consuming TRM anisotropy correction protocol was substituted by an alternative approach during the paleointensity experiments. Forty-two specimens, belonging to six samples, yielded highquality Thellier determinations. The NRM fraction f used for paleointensity determination ranges between 0.42 to 0.99, and the quality factor q (Coe et al., 1978) varies from 4 to 59, being normally greater than 5. These results correspond to data of good quality. The mean archeointensity values per pottery fragments range from 14.6±1.5 to 59.5±13.8 μT, while the corresponding virtual axial dipole moments range from 2.5±0.3 to 10.0±2.4 × 10 22 A m 2 . These new data, although not numerous, are of high quality and definitively contribute to the Mesoamerican, still insipient, archeointensity database.

No abstract

No abstract

No abstract

No abstract

Analysis of the aeromagnetic anomalies over the northwestern sector of the Tepic-Zacoalco rift documents a NE-SW pattern of lineaments that are perpendicular to the inferred NW-SE boundary between the Jalisco block and the Sierra Madre Occidental. The boundary lies within the central sector of the Tepic-Zacoalco rift immediately north of the Ceboruco and Tepetiltic stratovolcanoes and extends up to the San Juan stratovolcano, where it intersects the NE-SW magnetic anomaly lineament that runs toward the Pacific coast (which intersects two volcanic centers). This N35• E lineament separates the central rift zone of low amplitude mainly negative anomalies (except those positive anomalies over the stratovolcanoes) from the zone to the north and west characterized by high amplitude positive long wavelength anomalies. The NE-SW lineament is parallel to the western sector of the Ameca graben and the offshore Bahia de Banderas graben and to the structural features of the Punta Mita peninsula at the Pacific coast, and thus seems to form part of a regional NE-SW pattern oblique to the proposed westward or northwestward motion of the Jalisco block. The orientation of this regional structural pattern at the northern end of the Tepic-Zacoalco rift seems consistent with proposed dominant SW-directed extension along the rift during the Pliocene and Quaternary, rather than with NW-SE lateral strike-slip faulting. The orthogonal pattern that characterizes the northernmost boundary of the Tepic-Zacoalco rift is oblique to the pattern observed in the Grande de Santiago river (which conforms the northern limit of the rift) and for the central-eastern sectors of the Ameca graben (south of the rift). This spatial arrangement of major lineaments and structural elements points to a complex tectonic history for the region that includes the rifting of the Gulf of California and margin deformation due to plate convergence and kinematic re-organization events, and which may have resulted in distinct episodes of left-lateral, right-lateral and normal faulting in Neogene times, as proposed in recent studies.

Reportamos el estudio de paleointensidad de lavas históricas del volcán Paricutín por el método de microondas, las cuales fueron eruptadas durante el período entre 1943 y 1948. Muchas de las muestras se caracterizan por presentar gráficas ortogonales univectoriales. Los estudios de magnetismo de rocas y microscopía indican titanomagnetitas de dominio seudo sencillo como los portadores de la remanencia. La técnica de microondas se aplicó a muestras seleccionadas usando variantes al método de Thellier de Kono y Ueno (1977) y de Coe et al. (1978). Las muestras dieron resultados de paleointensidad de alta calidad técnica, no obstante mostraron alta dispersión en un mismo flujo siendo significativamente diferentes del valor esperado de 45 mT. Esto también se observó en los resultados obtenidos al aplicar el método de Thellier (Urrutia-Fucugauchi et al., 2004). Se exploraron las explicaciones a este comportamiento respecto a otros estudios en lavas históricas. La alta calidad técnica de los resultados sugiere que no hay criterios que puedan distinguir entre datos de paleointensidad correctos y erróneos.

No abstract